In stringed musical instruments such as guitars and bass guitars, a main component is the neck that provides or supports a fretboard or fretless fingerboard. The neck is typically made from wood and is ordinarily designed to be nominally flat along its length. When the instrument is strung and tuned, the high tension in the strings, in the order of one or two hundred pounds, sets up a strong continuous compressive stress in the neck that is unbalanced front-to-back in the direction that tends to bow the neck and cause concave curvature in an initially straight fingerboard.
The amount of concave curvature that occurs in the setup of a new neck depends on the strength of the neck material, and tends to increase over time due to the constant string tension and resulting neck pressure that is unbalanced front-to-back and is thus likely to cause neck bowing and concave curvature to an extent that makes the instrument difficult to play due to excessive string-to-fret separation along at least some portion of the fingerboard, known as high “action”, requiring excessive fingering force and string displacement in the player's technique.
In initial setup or refurbishing of a stringed musical instrument, the overall action is set by adjusting the height of the string-end support points, i.e. at the “bridge” and at the “nut” of the instrument. If the instrument has the conventional tension-adjustable truss member (commonly referred to a “truss rod” although it can be made with different cross-sectional shapes other than circular, e.g. rectangular, square, etc.), it can be tightened to counteract concave neck curvature, with maximum effect on the action height in the mid region of the fretboard/fingerboard length, so that, along with action height adjustment at the bridge and nut, this conventional instrument set-up system enables the action to be set up to optimize string-to-fret separation in three longitudinal regions of the fretboard/fingerboard: the two end regions and a mid-region.
In setting up high quality fretted stringed instruments, after the initial three point setup described above, any remaining anomalies in the action, e.g. between the three points, are usually subject to corrective work by the technician or luthier “dressing” and “crowning” the frets in the fretboard, i.e filing metal material from the fret tips and then re-rounding the fret tips.
While, historically, the foregoing setup procedure, i.e. three-point spacing adjustment plus dressing of individual frets, has worked tolerably well overall for the great majority of guitar-like stringed instruments, some new necks and many seasoned necks exhibit curvature patterns, such as non-symmetrical concave neck curvature that severely compromises proper setup with the available adjustment mechanisms. Even with a skilled technician or luthier, the work required often proves tedious and costly, or even impossible.
Furthermore, some types of high end stringed instruments such as the Chapman Stick (registered trademark), which is played by a unique variation of two-handed string tapping first discovered and taught by the present inventor, benefit particularly from very low “action” all along the frets as well as from moderate “relief” at the lower pitched frets. There is an unfulfilled need in the stringed instrument marketplace for a more versatile truss system that provides a degree of setup capability that goes beyond that of the usual single truss, e.g. by providing additional regions of adjustment along the fretboard/fingerboard.